1. Field of the Invention
This invention relates to magnetic disk drives. More particularly, the present invention relates to disk drive head stack and actuator arm assemblies that include a balanced weight that mechanically link selected actuator arms to increase the stiffness of the actuator and suspension assemblies.
2. Description of the Prior Art and Related Information
Magnetic disk drives, such as those used for mass storage in personal computer systems are well known. Generally, disk drives include a head disk assembly (HDA) and a controller printed circuit board (PCBA). The HDA includes a cover, a base, one or more disks, a head stack assembly (HSA) that includes an actuator assembly, head gimbal assembly (HGA) and a flex circuit. The HSA rotatably positions a slider having one or more transducers over a disk. A spindle motor spins the disks. FIG. 2 is a simplified side view of a conventional head disk assembly 200. The head disk assembly 200 includes a HSA that includes an actuator assembly 230 and a HGA 210. The actuator assembly 230 includes a body portion 240, a plurality of actuator arms 260 (four such actuator arms 260 being shown in FIGS. 2, 3 and 4) cantilevered from the body portion 240. The actuator assembly 230 also includes a voice coil motor (VCM), which is not shown in FIGS. 2–4. The HSA is pivotally attached to the base of the drive 216. Each HGA 210 is attached to a respective actuator arm 260 and supports a slider, such as shown at 241. A plurality of disks (only two disks 211 and 212 are shown in FIGS. 2–4), are clamped to a spindle motor (not shown in FIGS. 2–4) and are separated by spacers 206. The head disk assembly 200 of FIG. 2 is a constituent element of a so-called “depopulated” drive. In a depopulated drive, one or more disks are removed and replaced with an additional spacer, as shown in FIG. 2 at reference numeral 204. The middle spacers 206, 204 and 206 may be combined into a single spacer. Although the head stack assembly is configured to read and write to as many as three disks (each with two recording surfaces), the HDA 200 of FIG. 2 has been configured to read and write to only two disks 211 and 212. The two middle actuator arms 260 do not require a full complement of two HGAs 210. These unnecessary HGAs 210 may, therefore, be omitted to save on fabrication and assembly costs. As each actuator arm 260 is configured to support two HGAs 210 on opposite faces thereof, the absence of an HGA 210 from one face unbalances the actuator arm 260. To compensate for such missing HGA 210, weights 202 are conventionally attached to and cantilevered from the actuator arms 260 from which a HGA 210 is missing. The weights 202 are configured so as to have the same or substantially the same mechanical properties as the missing HGAs 210, such that the overall mechanical characteristics of the HDA 200 are unchanged.
FIGS. 3 and 4 show the mode shapes of the HSA at two different frequencies, which may manifest themselves during normal operation of the drive or during a shock event. FIG. 3 illustrates the case wherein the HSA undergoes a first bending mode in which the middle two actuator arms 260 bend in the same direction. The motion of the two middle actuator arms 260 shown in FIG. 3 is an in-phase motion, in that the arms 260 bend in the same direction at or near the same time. The bending of the arms in FIGS. 3 and 4 has been vastly exaggerated to better illustrate the mode shapes and the problems inherent in conventional depopulated drives. This first in-phase bending mode has been observed in current generation drives at about 906 Hz. FIG. 4 shows the same HSA experiencing an out-of-phase first bending mode, in that the actuator arms are bending in opposite directions. This first out-of-phase bending mode has been observed at an excitation frequency of about 943 Hz. As the actuator arms 260 are not and cannot be made to be perfectly stiff, these bending modes occur as the actuator arms 260 bend in response to a given excitation frequency or frequency range. Stiffening the actuator arms 260, all other aspects thereof remaining the same, tends to beneficially increase the frequencies at which the arms 260 bend and tends to reduce the amplitude of such vibrations. The stiffer the actuator arms 260 can be made, the higher the frequencies will be at which the actuator arms 260 will bend.
Such bending and torsion modes interfere with the drive's reading and writing activities, and typically slow down the drive's seek time performance. To address such bending and torsion modes, a notch filter or filters tuned to the bending and the torsion modes frequencies may be used in the drive's servo to attenuate signals at these frequencies, to the detriment of available servo bandwidth. Moreover, it is easier to attenuate higher frequencies without unacceptable loss of signal amplitude, as it is to attenuate unwanted bending and torsion modes frequencies at comparatively lower frequencies. From the foregoing, it may be appreciated that there is a clear need for shifting the bending and torsion modes frequencies higher and/or to eliminate one or more bending and torsion modes of actuator arms of hard disk drives. Doing so would decrease drive seek times, decrease the degradation of servo bandwidth caused by such bending and torsion modes, among other benefits.